The high concentrations of macromolecules within cells result in large excluded volumes, potentially causing very large shifts in rates and equilibria of reactions involving other macromolecules. Prior studies of such macromolecular crowding effects on DNA by ourselves and others have been restricted to empirical studies of model systems, leaving unanswered basic questions about DNA function under crowded conditions. We have addressed two of those questions. First we have studied the effective volume of DNA and related materials in crowding interactions. The two-phase distribution assay for volume occupancy described earlier was used to determine the effective volumes in crowding interactions of a series of small double- and single-stranded pieces of DNA. Their behavior is fully consistent with a simple theoretical treatment, the available volume model, used for predicting excluded volume effects. These results are "in press" in Biopolymers. Second, we are currently measuring the crowding effects of actual cellular material, cytoplasmic extracts from E. coli, on a DNA test system. The test system measures the rate of in vitro cohesion of restriction fragments of lambda DNA. Preliminary results indicate accelerations of at least 1-2 orders of magnitude. In addition to these experimental studies, I am writing a review on macromolecular crowding in collaboration with Allen Minton (also of NIDDKD) for the 1992 edition of Annual Reviews of Biophysics and Biophysical Structure.